In many radio communication systems, such as local multipoint distribution systems (LMDS), point to multipoint systems or time division multiple access (TDMA) burst systems, a number of independent subscriber unit data transmitting and receiving stations have access to a single base station. Subscriber unit transmissions are managed on a time-division basis such that the available base station time is allocated among the subscriber units in some manner. The performance of these systems depends to a large degree on the efficiency with which the functions of rapid synchronization and demodulation are performed.
With conventional burst mode communications, there is the problem of frequency synchronization of the base station receiver to the subscriber unit's return channel. The problem is caused, at least in part, by noise or other uncertainty concerning the received signal's frequency. The received signal uncertainty causes the base station to spend an excessive amount of time synchronizing before the base station can successfully extract data from the received signal. Excessive synchronization or acquisition time translates into low operating efficiencies. Such low operating efficiencies are exacerbated in burst mode communication systems because the synchronization time is repeated for each burst. Systems having a larger number of brief transmission bursts are less efficient than systems which accommodate a smaller number of longer bursts due to the overhead time to synchronize to the greater number of bursts.
In digital communication systems, at least two diverse types of frequency synchronization are conventionally performed in receivers. Carrier synchronization refers to a process wherein a frequency of a receiver oscillator is adjusted, typically using a voltage controlled oscillator (VCO) within a feedback loop located in the receiver, to match the frequency of a received signal, either in RF or IF form. Baud synchronization, also called bit synchronization, bit timing, and the like, refers to a process for adjusting a different oscillator to determine the baud of data conveyed by a received signal. Baud synchronization is conventionally performed using a feedback loop located in the receiver. In a typical digital communication system, carrier synchronization is achieved before baud synchronization can be achieved. Both types of synchronization is achieved before data is successfully demodulated.
In each of these two types of frequency synchronization, the further the frequency of the received signal is offset from the initial frequency of the receiver's internal oscillator, the longer the synchronization process takes. In addition, variable frequency oscillators, such as VCOs, introduce phase noise. When feedback loops attempt to track signals exhibiting large phase and thermal noise, an undesirable compromise is reached between narrow and wide loop bandwidths. A narrow loop bandwidth may reduce the noise-induced degradation and achieve the lowest possible bit error rates, but synchronization times increase as bandwidth decreases and tracking is doubtful. A wide loop bandwidth may successfully track large amounts of phase noise and quickly synchronize, but a portion of the phase noise passes through and increases bit error rates.
It will be appreciated from the foregoing that there has been a need for an improved method of synchronization in radio communication systems.